An organic electroluminescence device is a spontaneous light emitting device which utilizes the phenomenon that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang et al. of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Page 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used tris(8-quinolinolato)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excitons which are formed by blocking and recombining electrons injected from the cathode can be increased, and that the excitons formed in the light emitting layer can be confined. As described above, for the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron-transporting light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer, and an electron-transporting (injecting) layer are well known. In order to increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material, chelate complexes such as tris(8-quinolinolato)aluminum, coumarin derivatives, tetraphenylbutadiene derivatives, bis-styrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials, and development of a device exhibiting color images is expected (For example, Patent Documents 1 to 3).
In recent years, a large number of investigations have been conducted on the use of a phosphorescent compound as a light emitting material and the use of energy in a triplet state in EL light emission. A group of Princeton University has reported that an organic light emitting device using an iridium complex as a light emitting material shows high luminous efficiency (Non-patent Document 1). In addition to the organic electroluminescence device using a low molecular weight material as described above, an organic electroluminescence device using a conjugated polymer has been reported by a group of Cambridge University (Non-patent Document 2). In this report, light emission has been confirmed from a monolayer of polyphenylene vinylene (PPV) formed in a coating system.
Recent advances in organic electroluminescence device are remarkable, and characteristics of the organic electroluminescence device allow formation of a thin and lightweight light-emitting device with high luminance under application of a low voltage, wide range of emission wavelengths, and high-speed response, thereby suggesting the possibility of extensive uses.
In association with the significant progress of an organic light emitting device, performance requested of a light emitting material has been growing, and Patent Documents 4 and 5 each disclose a compound with a specific structure as a material achieving high luminous emission under application of a low voltage and being excellent in durability.
Further, Patent Document 6 discloses a material to which a specific flexible partial structure such as an ortho bond is introduced and organic EL device with the use of it. However, regarding with a device performance, despite the improvement of the color purity, the lifetime is too short to achieve the practical use.
In the present state of affairs, however, an optical output of further high luminance or high conversion efficiency is necessary. Moreover, there are many problems of durability such as change with the passage of time due to a long time usage and degradation or so caused by an atmospheric gas including oxygen or a moisture. Furthermore, although light emission of blue, green and red with excellent color purity becomes necessary considering about application to full color display, countermeasures about these problems are not sufficient yet.                Patent Document 1: JP 08-239655A        Patent Document 2: JP 07-183561A        Patent Document 3: JP 3-200889A        Patent Document 4: JP 2004-83481        Patent Document 5: JP 2004-43349        Patent Document 6: US 2005/0089715 A1        Non-patent Document 1: Nature, 395,151 (1998)        Non-patent Document 2: Nature, 347,539 (1990)        